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March 2006

Surface and Interfacial Tension

One important task for which the ramť-hart goniometer is often employed is the measurement of surface and interfacial tension. Two of our models (Model 250 and 500, see below) have the ability to perform surface tension analysis (in addition to contact angle measurement and surface energy calculations) out of the box. Our other models (120, 200, and 300) can be upgraded in order to perform such analysis.

Prior to the introduction of our DROPimage Advanced software which uses state-of-the-art drop shape analysis, there were several methods that are frequently employed to measure surface tension. The Du NoŁy ring method requires the use of a precise scale which measures the force exerted to lift a solid ring (i.e., torus-shaped) from a pool of test liquid. The math for this method requires the radius of the ring as well as the thickness of the ring. This method is quite problematic and difficult to implement. The ring must be kept constantly parallel with the liquid surface. The liquid surface must also be kept absolutely motionless. Contaminants in the liquid and on the solid phase can adversely affects the results. A correction factor must be added to take into account the small amount of liquid that is lifted up with the ring. Lastly, it's often difficult to make a valid measurement before the meniscus breaks. Prior to the advent of software-driven tools that can perform drop shape measurements, the manual Du NoŁy ring device was the primary method for studying surface tension. Overall, it is difficult to obtain accurate data using this method.

Another method that has been used for measuring surface tension is the Wilhelmy Plate Method. This method requires a plate to be partially submerged vertically in the test liquid. The plate is then lifted out and the height of the lamella (or meniscus) and the contact angle formed between the plate and the liquid is measured. A variation of this method requires that the liquid level be raised while simultaneously measuring the change in force. Like the Du NoŁy ring method, the Wilhelmy Plate Method is fragile and subject to a number of vagaries. The solid's surface properties must be uniform and the plate must be absolutely clean. For these reasons, ramť-hart does not offer an instrument that uses either the Du NoŁy or Wilhelmy methods.

The newest widely-accepted method which is gaining widespread popularity among researchers and in industry is the drop shape method. Our DROPimage Advanced software employs drop shape analysis and allows for studies that use a pendant drop, an inverted pendant drop, a sessile drop, or a captive bubble. A drop in a gas phase or a gas bubble in a liquid is used to measure surface tension while two liquids are required to measure interfacial tension.

There are four primary advantages to the drop shape method over traditional ring and plate methods: First, the drop shape method can be done very quickly and without extensive apparatus and setup time -- such as silk strings, plates, and rings. With the ramť-hart goniometer, a drop is dispensed from a needle and the DROPimage software captures the image and in real-time performs the math to calculate the drop shape and measure the surface tension. Second, the drop shape method is not so sensitive to contaminants on the solid phase as this is much smaller compared to the plate and ring methods. While contaminants in the liquid can have an adverse effect using any method the cleanliness of the needle is not as critical compared with the requirement with the Du NoŁy ring and Wilhelmy plate be absolutely clean. It's also much easier to keep the liquid phase uncontaminated in a syringe compared with the open baths required by the ring and plate methods. Third, the drop shape method requires as few as 10uL of fluid while the ring and plate methods require a large bath (ml's) of fluid. And fourth, the ramť-hart Advanced Goniometer with DROPimage Advanced (our Model 500) is more affordable than any software-driven tool on the market that employs the ring and plate methods. We offer an array of modular accessories such as chambers and fixtures and a software-driven dispensing system that can be setup for any particular requirement. Overall, you will find the drop shape method employed by our DROPimage software to be highly reliable and flexible for myriad applications.

If you are interested in learning more about the drop shape method used in our DROPimage Advanced software, please contact us to request a copy of our technical paper which describes the physical and mathematical methods used in DROPimage Advanced. 

Model 250 and 500 Goniometer and Tensiometer
The ramť-hart Model 250 is both a Goniometer and a Tensiometer. That is, it has the ability to measure both surface energy (using six different tools, see February 2006 Newsletter) as well as surface tension (see above section). Model 250 is suited to research environments where cost constraints preclude the full-featured Model 500. The 250 supports the optional software-driven Automated Dispensing System, the Environmental Fixture (ideal for captive bubble and liquid/liquid studies), the Tilting Base, Film Clamps, and small Wafer Support (up to 3"). Both the Model 250 and 500 ship with DROPimage Advanced software.

The Model 500, shown below, is a full-featured top-of-the-line tool supporting, in addition to the options supported by the 250, our Environmental Chamber, Chamber with Humidity Control, High Pressure Chamber, Elevated Temperature Syringe, and Wafer Support up to 12" in diameter. In addition to our standard accessories, custom fixture and supports can be made to meet your requirements.

Model 500 with Optional Wafer Support

Both the 250 and 500 use the same proprietary variable control fiber optic backlighting subsystem and CCD camera imaging system. Both tools use a precision tubular rail system for movement on the Y-axis and both ship with our micro-syringe fixture and assembly, PC, and shade kits -- everything needed to get started. The 250 differs from the 500, in addition to a shorter rail system, by employing a 3-axis locking stage capable of supporting up to 3.55lbs (1.61kg). The Model 500 stage offers both fine and coarse vertical adjustment and can support up to 30lbs (13.6kg) -- making it a better choice for more advanced applications that require heavy chambers or large custom, lead frame, or wafer supports. The Model 500 is based on the same frame used by our classic Model 100 which was originally developed over 40 years by Dr. William Zisman at the Bethesda (Maryland) Naval Research Lab.


We appreciate your comments, your continued business and look forward to working with you in the near future. If you would like more information on any of our products, or a copy of our current price list, please don't hesitate to contact me personally.  



Carl Clegg
Director of Sales
Phone 973-448-0305
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